This invention relates to acoustic silencers capable of attenuating noise associated with a gaseous medium passing through the silencer or duct.
Silencers for air ducts and air delivery systems are known with conventional silencers generally employing sound attenuating fibrous material such as fibreglass bats. Generally, in a conventional silencer, which may be constructed to replace a section of a duct, the interior sidewalls that form the sides of the air passageway are made of perforated metal sheets having a large number of holes distributed over their surface. Behind each perforated sheet there is placed a fibrous, acoustically absorptive bat of material which may comprise fibreglass, as indicated or rock wool or even foam. In order to hold this bat of material in place there is normally also an exterior sheet metal wall which is imperforate. Preferably the interior perforated sheet is made to provide optimum sound access from the air flow passage to the sound attenuating material. For example, the open face area of the interior sheeting can be 20% or more.
However, it is recognized that these conventional silencers do result in some problems, particularly when used for certain applications. For example, the fibrous material of the packing can erode when the gas or air is flowing at high velocity, thus making the silencer eventually less effective. Also, the fibrous material is known to absorb toxic or flammable substances or even undesirable microorganisms with the passage of time. In the event of fire, some types of acoustic packing material may provide fuel for the fire or may give off toxic gases.
So called "packless silencers" are also known in the heating, ventilating and air conditioning industry. One form of packless silencer is described in U.S. Pat. No. 4,287,962 issued Sep. 8, 1981 to Industrial Acoustics Company. In one version of this silencer, there is a four sided silencing duct having a pair of opposed facing panels with a generally flattened semi-elliptical shape. The opposing flat portions of each panel are perforated sheets and the holes in these sheets open to chambers formed behind each panel. These chambers are separated by acoustically opaque and fluid impervious dividers or partition walls. Also, the facing panels have curved end portions which are non-perforated and thus acoustically opaque. The facing panels are made from galvanized or stainless steel or even from a non-metallic (but stable) material. The open area created by the perforations in the centre wall portions is in the range of 2 to 10% and the perforations are required to have a diameter in the range of from 0.032 inch to 0.125 inch with the panels having a thickness of from 26 gauge to 11 gauge.
It is an object of the present invention to provide an improved and alternative form of acoustic silencer suitable for use in a duct system, which silencer does not require the use of fibrous packing and which can be constructed to be highly effective at reducing the noise level passing through the duct.
It is a further object of the present invention to provide an improved acoustic silencer which employs a combination of perforated sidewalls and one or more screen layers extending along outer surfaces of the perforated sidewalls, with enclosed chambers being formed behind the perforated sidewall and screen.
It is a further object of the present invention to provide an improved acoustic silencer for use in a airflow duct for attenuating noise which does not employ fibrous sound attenuating material and which at the same time is relatively simple to construct and manufacture and requires little maintenance.